Information is more rapidly processed and transmitted using optical as opposed to electrical signals. There exists a need for finding nonlinear optical materials which alter the transmission of optical signals or serve to couple optical devices to electrical devices, i.e., electro-optic devices.
Second order nonlinear optical (NLO) polymers are expected to find extensive uses in opto-electronic applications. NLO polymers have several advantages over single crystalline inorganic and organic molecular systems. These include easy preparation, adjustable refractive indices and controlled arrangement of spatial order. For second order applications, it is imperative that the material be noncentrosymmetric. In noncentrosymmetric organizations, several organic molecular and polymeric systems have been characterized by large second order NLO coefficients, ultra-fast response times, performance over a broad wavelength range and high laser damage threshold compared to the traditional inorganic materials, e.g., lithium niobate (LiNbO.sub.3) or potassium dihydrogenphosphate (KH.sub.2 PO.sub.4). Background information relating to the principles of nonlinear optical polymers, can be found in Nonlinear Optical and Electroactive Polymers, edited by Prasad and Ulrich, Plenum Press (1988).
A number of applications, such as second harmonic generation (SHG), frequency mixing, electro-optic modulation, optical parametric emission, amplification and oscillation have been proposed for organic and polymeric materials with large second order NLO coefficients. A number of approaches have been made in the past decade to organize NLO molecules in a polymer matrix in a noncentrosymmetric manner. The most important, but not the only aspect from the standpoint of application, is the organization of NLO molecules into preferred orientation and their stability in the aligned state up to at least cold wire bond temperatures (about 100.degree. C.).
Historically, one of the first approaches to this alignment of NLO molecules in a polymeric system came with the concept of the guest-host system. (Singer, 1986). The NLO molecules may be incorporated by a solution casting method with an amorphous polymer and the second order non-linearity may be imparted by subsequent poling of the NLO molecules in the matrix using an external electric field, e.g., corona poling, parallel plate poling or integrated electrode poling. Advantages of this approach are ease of processing, tailorable refractive indices, control of spatial ordering of the polymer, and choice of a wide range of materials. However, the decay (both the initial and long term) of second order properties as confirmed through SHG from the matrix is unavoidable when the poling field is withdrawn from the matrix. Moreover, a high degree of loading of the NLO molecules in the polymer is not possible because of phase segregation of the matrix or blooming of NLO molecules in the matrix, both resulting in optical scattering.
Electro-optic waveguide devices form an essential component of the emerging field of integrated optics. Electro-optics is a property whereby materials change their refractive index upon the application of an electric field. This change in refractive index affects the way the material interacts with light. Electro-optics and electro-optic waveguide devices are described, for example, by Nishihara (1985), and by Hunsperger (1985).
Electro-optic waveguide devices can be passive waveguide devices or functional waveguide devices. Some passive waveguides are optical beam-dividers, polarizers, and the like. Some functional waveguides are phase modulators, Mach-Zehnder modulators, and the like. Generally, electro-optic waveguides, or optical waveguides in short, consist of a transparent waveguiding core ("guiding layer") surrounded by a layer of transparent materials ("cladding layer"). Among these layers, the guiding layer serves the important function of interacting with and affecting the propagation of light. Materials that form the guiding layer have been traditionally inorganic materials such as lithium niobate, potassium dihydrogen phosphate, ammonium dihydrogen phosphate, and the like. These are typically single crystal materials, and lack processing capabilities. In recent years, NLO polymeric materials have seen increased application as guiding layers. Generally, polymeric NLO materials can or may have the specific advantages mentioned above of fast response time, small dielectric constant, good linear optical properties, large nonlinear optical susceptibilities, high damage threshold, engineering capabilities, and ease of fabrication.
There are various known polymeric organic materials which possess specific nonlinear optical properties and various known processes for making such polymeric organic materials. Many of the current polymeric organic materials prepared by the prior art are prepared by blending a NLO molecule into a polymer host material. "Blending" herein means a combination or mixture of materials without significant reaction between specific components.
As mentioned above, a problem associated with a "guest-host" polymer with NLO properties produced by simply blending of NLO molecules into a host polymer is that these polymer materials lack stability of orientation. Generally, the incorporation of molecular structures which have NLO activity into the backbone of a polymer chain will decrease the likelihood of the structural reorganization in comparison with polymers in which the NLO active molecule is simply blended. It is therefore desirable to provide a polymer material with NLO groups covalently bonded to the backbone of the polymer material to minimize relaxation effects.
There is a continuing effort to develop new nonlinear optical polymers with increased nonlinear optical susceptibilities and enhanced stability of nonlinear optical effects. Generally, nonlinear optical polymers contain nonlinear optical moieties as covalently linked part of polymer chains. Examples of such polymers are described in Williams, ed. (1983). The nonlinear optical moiety may be part of the polymer backbone, or it may be appended to the polymer backbone through intervening spacer groups. The latter are referred to as side chain nonlinear optical polymers. EP 89402476.9, for example, discloses nonlinear optical polymers where the nonlinear optical moiety forms part of the polymer backbone. U.S. Pat. Nos. 4,779,961; 4,801,670; 4,808,332; 4,865,430 and 4,913,844, the teachings of which are herein incorporated by reference, disclose several side-chain nonlinear optical polymers.
Nonlinearity of moieties is described in terms of second order nonlinearity, third order nonlinearity, and so on, with the corresponding unit values being referred to as second order nonlinear optical susceptibility, third order nonlinear optical susceptibility, and so on. Nonlinear optical moieties of polymers that are preferred as guiding layers in optical waveguide devices generally must possess acceptable second order nonlinear activity. These moieties are generally made up of conjugated pi-electron systems with an electron donating group such as an amine group, and an electron-acceptor group such as a nitro group forming either end of the conjugated pi-electron system.
Nonlinear optical polymers can be cast as films on substrates by processes such as spin coating from a solution of the polymer in a solvent, spraying, Langmuir-Blodgett deposition, and the like. The substrate materials employed for electro-optic waveguide devices are generally inorganics such as silicon, GaAs, GaAlAs and the like. Silicon is particularly preferred as substrate material due to its ready availability in wafer form in a well-purified state, and the highly-developed state of its technology in integrated circuit and electronics industries. Wafers from silicon also have the advantage that they can be easily cleaved into minute chips carrying the individual devices.
Although second order nonlinear optical (NLO) polymers hold promise for practical applications in electro-optical devices, a number of issues remain have to be addressed before they can see wider commercial application. (Prasad, 1991; Marder, 1991; Chemla, 1987; Williams, 1984.) Three of these crucial issues are the high temporal stability of dipole orientation, large optical nonlinearity and minimum optical loss. Due to a realization of the intrinsic nature of the optical loss (due to C-H overtone vibration absorption), major research efforts have been focused on optimizing the optical nonlinearity and stabilizing the dipole orientation.
Different approaches have been taken to address these issues, and considerable progress has been achieved. For example, various cross-linking schemes (photochemical and thermal cross-linking) have been developed to lock the dipole orientation in the polymer matrix after electric poling. Temporal stabilities of second order NLO activity thus have been enhanced. The rationale behind the design of these polymers is that after cross-linking, the motion of the free volume in the polymer matrix can be frozen. This is reflected in the increase in glass transition temperatures of the resulting materials. The same notion leads to the concept that as long as a polymer has a high glass transition temperature, the induced dipole orientation can be stabilized in a certain temperature range. This was clearly demonstrated in second order NLO polyimide composite materials.
Wu et al. (1991 ) have found that polyimide composite materials prepared by electric poling and thermal curing from polyamic acid doped with an Eriochrome black T dye exhibited long term NLO stability at high temperature. More recently, Marks et al.(1992) and Dalton et al. (1993) developed a different approach to synthesizing polyimide second order NLO materials, realizing significant enhancement in stability due to high glass transition temperatures. The present invention represents a new nonlinear optical chromophore which allows the synthesis of a new polyamic acid (see Scheme I, FIG. 1). This polyamic acid can be easily cast into optical quality films and be imidized by thermally curing to generate polyimide with a high glass transition temperature. Very large and exceptionally stable second harmonic generation (SHG) coefficients were observed.